(a) Field of the Invention
The present invention relates to a light emitting display device using a demultiplexer. More specifically, the present invention relates to power wiring of a light emitting display device using a demultiplexer.
(b) Description of the Related Art
A display device needs a scan driver for driving scan lines and a data driver for driving data lines. The data driver has as many output terminals as the number of data lines in order to convert digital data signals into analog signals and apply them to all the data lines. In general, the data driver is configured by a plurality of integrated circuits (ICs). A plurality of ICs are used to drive all the data lines since the number of output terminals had by a single IC is limited. Hence, demultiplexers are adopted so as to reduce the number of data drive ICs.
For example, a 1:2 demultiplexer receives data signals that are time-divided and applied by the data driver through a signal line, divides them into two data groups, and outputs them to two data lines. Therefore, usage of a 1:2 demultiplexer reduces the number of data drive ICs by half. Recent liquid crystal displays (LCDs) and organic electroluminescent displays are beginning to mount the ICs for the data driver on the panel, and in this instance, there is a greater need to reduce the number of data drive ICs.
FIG. 1 shows a simplified block diagram of a conventional organic electroluminescent (EL) display using a demultiplexer. When the IC for the demultiplexer, the data driver, and the scan driver are manufactured to be directly mounted on the panel, power supply points, power supply lines, and power wiring are formed as shown in FIG. 1 to supply power to pixels. Scan driver 20 for applying select signals to select scan lines SE1 to SEm is provided on the left of display area 10, and scan driver 30 for applying signals for controlling light emission to the emit scan lines EM1 to EMm is provided on the right thereof. Scan driver 30 can be removed when the pixels do not use signals for controlling light emission. Demultiplex unit 40 and data driver 50 for applying data signals to data lines D1 to Dn are provided on the bottom of display area 10. Vertical lines 60 are formed in the vertical direction to supply power supply voltages to the respective pixels. Power cable 70 coupled to vertical line 60 on the top of the substrate is formed in the horizontal direction. Power cable 70 and external power supply cable 80 are coupled through power supply point 90. Power supply cable 80 surrounds the two scan drivers 20, 30 and accesses an external power source through a pad (not shown) formed on the bottom of the panel.
FIG. 2 shows a simplified circuit diagram of a pixel circuit of an organic EL display. The basic pixel circuit uses two transistors M1, M2, and does not use emit scan lines EM1 to EMm. In the pixel circuit of FIG. 2, when switching transistor M2 is turned on in response to a select signal from select scan line SE1, the data voltage from data line D1 is applied to a gate of driving transistor M1. A source-gate voltage at transistor M1 is stored in capacitor C1, and the current from driving transistor M1 is applied to organic EL element OLED in correspondence to the stored voltage, thereby displaying images.
Accordingly, the current is supplied to the OLED from power supply voltage VDD while the images are displayed in the pixel circuit of the organic EL display device. That is, voltage dropping is always generated because of the parasitic resistance provided on the wires since the current flows to vertical lines 60, power cable 70, and power supply cable 80 coupled to power supply voltage VDD while the images are displayed. Magnitudes of power supply voltage VDD are varied by the voltage dropping according to the position of the pixel circuit arranged along power cable 70 and vertical lines 60 from power supply point 90. Accordingly, the source-gate voltage at transistor M1 becomes different according to the position of the pixel circuit, the magnitude of the current supplied to the OLED becomes different, and the brightness becomes varied according to the position of the pixel circuit.
U.S. Pat. No. 6,229,506 issued to Dawson and U.S. patent publication No. 2002/0033718 of Tam disclose pixels circuits for compensating for the voltage dropping. The Dawson patent discloses a pixel circuit for using voltage to program the voltage to capacitor C1 (referred to as a “voltage programming pixel circuit” hereinafter). The publication by Tam discloses a pixel circuit for using current to program the current to capacitor C1 (referred to as a “current programming pixel circuit” hereinafter). These circuits compensate for the source-gate voltage at a driving transistor stored in the capacitor by modifying the gate voltage at the driving transistor by as much as the source voltage at the driving transistor is varied by the voltage dropping. However, such circuits only compensate for the source-gate voltage at a driving transistor and fail to compensate for a margin needed for forming an operational point of the driving transistor.
In more detail, referring to FIG. 3, in the current programming pixel circuit the characteristic curves between the current and the drain voltage of the driving transistor according to the source-gate voltage of the current driving transistor at the time of emitting the light by the organic EL element are given as {circle around (1)}, {circle around (2)}, {circle around (3)} and {circle around (4)} of FIG. 3, and the characteristic curve between the current flowing through the organic EL element and the corresponding anode voltage of the organic EL element OLED is given as L1. The respective characteristic curves {circle around (1)}, {circle around (2)}, {circle around (3)} and {circle around (4)} in FIG. 3 correspond to the different source-gate voltages of the driving transistor. The current programming pixel circuit stores the voltage corresponding to the current flowing to the driving transistor, and allows the organic EL element to emit light through the current flowing to the driving transistor by the voltage stored in the capacitor, thereby compensating for the deviation of the transistor.
In this instance, operational point P is determined at the crossing point of the characteristic curve of the organic EL element, the characteristic curve of the driving transistor, and operational point P is to be established with a predetermined margin in the saturation region of the characteristic curves since it is impossible to compensate for the deviation of the driving transistor when operational point P digresses from the saturation region in the current programming pixel circuit. Since the margin is narrowed as the current flowing to the organic EL element is increased, a predetermined margin Mg is to be occupied at maximum current Imax of the organic EL element.
When voltage dropping is generated at the power supply voltage, the characteristic curve of the driving transistor is moved to the left by magnitude Vd of the voltage drop, and operational point P is formed out of the saturation region. Accordingly, the characteristic curves of the driving transistor and the organic EL element are not compensated. Power consumption is increased since the difference between power supply voltage VDD and a voltage VSS coupled to a cathode of the organic EL element needs to be increased in order to occupy the margin in consideration of the voltage drop.